Precious metals and in particular silver may be found in solution from which recovery of the metal is desired. Silver is commonly dissolved in spent solutions of photographic fixer or waste hypo solution. Recovery is desired for the value of the silver as well as to prevent discharge of heavy metal as a pollutant into waste water systems.
A well accepted method of recovering silver or other precious metal is by replacement reaction wherein a metal higher in the electromotive force series that the metal to be recovered is contacted with the solution, causing the precious metal to be reduced and thereby come out of solution. The precious metal may be recovered as a sludge or precipitate or as a plating on the metal of higher electromotive force.
The replacement reaction concept has been embodied in numerous methods and types of equipment. Batch processing is known wherein silver-containing solution is mixed with powdered metal and agitated to produce a relatively complete reaction. Continuous flow processing is known wherein a silver-containing solution is passed through a bed or matrix of fine metal, with steel wool, scraps of screening, and even sections of automobile radiator cores having been used to supply the metal of higher electromotive force. Other apparatus has employed the replacement metal in more massive form such as in a grate, perforated plate, stack of spaced plates or in a metal cylinder about which the silver-containing solution is caused to circulate. Still other apparatus has employed a cylindrical roll of window screening in combination with a canister that directs the silver-containing solution radially through the roll, whereby the replacement metal is in a massive structure that maintains a reasonably large surface area per unit of volume.
Another proposed structure for the replacement metal is to use powdered metal dispersed on a suitable medium such as alumina or other porous inert substance, whereby the powdered metal has a large surface area for a high reaction rate while deriving support structure from the alumina.
The primary problems encountered with replacement reactions are incomplete reaction and clogging. Batch processing can overcome these problems, but this solution is inconvenient and may require considerable chemical skill in order to determine the approximate silver content of the batch and thereby enable the appropriate quantity of replacement metal to be added to the batch. Continuous flow processes utilizing a massive form of replacement metal tend to be both slow and inefficient, since the surface area of available replacement metal is quite small per unit of volume. If the massive elements are closely spaced, clogging can occur, while if the elements are more widely spaced, substantial portions of the silver-containing solution may pass through the elements without sufficient contact for good recovery of available silver. Fine metal beds are also susceptible to clogging by precipitated solids; and in the case of steel wool or the like, it has been observed that sections of the wool may be depleted by the replacement reaction, leaving flow channels where unreplaced silver solution can pass through the apparatus.
The silver precipitated in a replacement reaction may be recovered in a variety of ways. If the flow of hypo solution is by gravity and not particularly turbulent, the precipitate will settle out in the reaction vessel. More positive methods of recovery include the use of a filter to capture the silver metal. It has been proposed to place the replacement metal in a fabric bag so that the precipitate will be retained in the bag. Chemically inert fibrous pads have also been used as a filter at the outlet of a reaction vessel, with compressed fiberglass filaments having been used for this purpose. In at least one instance, it has been proposed to interleave layers of replacement metal with layers of filter material so that the replacement or exchange reaction takes place in one zone and the reaction product is accumulated in a physically separated second zone; and it has further been alleged that such an interleaved arrangement of zones is beneficial when different replacement metals are in respectively neighboring replacement metal zones. Thus, when a filter other than the replacement metal itself has been used, the filter has been located in a physically separated zone from the replacement metal.
While the use of filters can increase the yield of the silver recovery process and permit the use of pumps to rapidly circulate the silver-containing solution, the filter has remained in a position where it initially contacts the liquid at a zone interface where the natural tendency is for the face of the filter to accumulate the majority of the captured product and, accordingly, the filter is subject to clogging at the zone interface.